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Spread Spectrum and CDMA |
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parallel to the in-phase and quadrature branches of the modulator to be multiplied by the binary PN-I, PN-Q codes (see Section 11.3.2) and shaped in the frequency domain by the baseband filters. The multiplication of the in-phase and quadrature signals by cosine and sine CW components of frequency f0 with their subsequent summation performs up-conversion of the BS signal and finishes the modulation process. As is seen, the input baseband signal in both branches is the same. As such, it is a sum of multiple binary voltages, i.e. is multilevel real. Assume for a while that there is only a single physical baseband channel fed immediately to the modulator branches without summation with other channels. We may postulate that each physical channel is processed this way, i.e. there are as many modulator branch pairs as channels and the outputs of all these parallel modulators are added up coherently. Since the scheme of Figure 11.4 is linear, and hence the superposition principle is valid, its output effect is identical to that of the hypothetical scheme above, i.e. with individually modulated channels. That is why we may say that in the forward link of IS-95 DS spreading is used where the binary datastream (channelized by a Walsh function) modulates the QPSK spreading code (see Section 7.1). Since the rate of the codestream at the modulator input is 19.2 kbps, each symbol has duration covering 64 short code chips. Hence, the forward link spreading factor is 64.
It is worth noting that the long code plays no role in DS spreading of the forward link signal, taking part only in data encryption and power control bit positioning. It is often said that forward link spreading is done by both Walsh codes and short PN-codes. Yet, conceptually classifying Walsh functions as channelizing and PN-codes as spreading may look more convenient.
11.3.3.6 MS processing of forward link signal
Signal processing in the MS receiver rests on the classic procedures discussed in depth in the previous chapters. On successful acquisition of a pilot signal, the receiver DLL pulls in and continuously tracks the short code of the contacted BS. The local replica of the short code produced by DLL serves for despreading the received signal. The outcome of the despread pilot channel is a ‘pure’ CW carrier down-converted to appropriate intermediate frequency. A phase-locked loop tunes the local crystal oscillator to be coherent with this CW signal, providing thereby the reference for coherent data demodulation. After demodulation and deinterleaving the data transmitted over synchronization, paging and traffic channels are separated from each other in correlators using Walsh-sequence references, decoded by the Viterbi algorithm and used according to their destination. For example, a digital-to-analog converter transforms speech data of the traffic channel into voltage, which becomes audible with the aid of an earphone.
Every MS receiver contains several (four or more) parallel channels capable of searching and tracking the pilot signal. One goal of it is arranging the RAKE receiver, which realizes the multipath diversity benefit of spread spectrum (see Section 3.7). Typically at least three such channels are used to implement RAKE fingers. Another procedure requiring autonomous pilot signal channels in the MS receiver is handover. A reserve correlator (or set of them) performs permanent scanning of the time domain, trying to determine if signals of other BSs are present, possibly more intense and